The WP4C soil water potential instrument is manufactured by METER Group (formally Decagon Devices).

The WP4C advances laboratory measurements of soil water potential and the water potential of any substrate or material.

It supersedes the old technology of the pressure chamber because it is safer, quicker, cheaper, and measures a wider range of water potential values.

Use the WP4C to measure water potential in 5 to 10 minutes.  Range: -0.1 to -300 MPa*. A complete soil water characteristic curve, also called a soil moisture retention or release curve, can now be completed in a matter of hours rather than days to weeks with the old pressure chamber technique.

The WP4C dew point Hygrometer is a fast, accurate way to measure water potential in any porous material. Typically used to analyze soil, soilless substrates, plant material and seeds, it has been used to measure a variety of unusal things, including apples, bricks and insect carapaces.

easy operation

Simply place the sample in a round sample cup (4cm in diameter and 1cm tall) and set the cup in the lexan sample drawer, slide in the drawer and turn the knob to seal the chamber.  After about 5 minutes, the WP4C will beep and show your final water potential reading.

precise mode

Measurements can be inaccurate if they're made too soon.  When run in Precise Mode, the WP4C checks to make sure the sample has reached equilibrium before displaying a final reading.

faster equilibration

The WP4C's nickel Teflon sample chamber coating is hydrophobic.  Less water absorption in the sample chamber means faster equilibrium.

finely tuned calibration

The WP4C sensor is durable and easy to clean.  The instrument can be calibrated using saturated salt solutions (mix your own or order single-sample vials of standards on the accessories tab).  New algorithms let you fine-tune calibration for excellent accuracy (±0.05 MPa or better).

data evaluation software

The HYPROP FIT Software can be used to plug in water potentail data collected by the WP4C for fitting different water retention models (i.e. van Genuchten, van Genuchten Bimodal, Brooks & Corey).

How Does it Work? Chilled Mirror Dew Point Technique

The WP4C measures water potential by determining the relative humidity of the air above a sample in a closed chamber (an AOAC-approved method; also conforms to ASTM 6836).  Once the sample comes into equilibrium with the vapor in the WP4C's sealed chamber, the instrument finds relative humidity using the chilled mirror method.  This method entails chilling a tiny mirror in the chamber until dew just starts to form on it.  At the dew point, the WP4C measures both mirror and sample temperature with 0.001°C accuracy. This allows the WP4C to deliver water potential readings with unparalleled accuracy in the -0.1 MPa to -300 MPa range

limitations

The WP4C range goes to 0 MPa, but readings of samples wetter than -0.1 MPa will have an increasing, and typically unacceptable, percentage of error.  Some users may be able to make useful measurements in samples wetter than -0.1 MPa using special techniques.  For more information, see the WP4C User Manual.

At high water potentials, the temperature differences between saturated vapor pressure and the vapor pressure inside the sample chamber become vanishingly small.  Because of this, the WP4C can now resolve temperatures to a thousandth of a degree.  To read water potentials in the wet range (0 to -0.1 MPa), we recommend German manufacturer UMS's fast response tensiometers and HYPROP moisture release curve instrument.

*Note: The WP4C range goes to 0 MPa, but readings of samples wetter than -0.1 MPa will have an increasing, and typically unacceptable, percentage of error.  Some users may be able to make useful measurements in samples wetter than -0.1 MPa using special techniques.  For more information, see the WP4C User Manual.